In the past several years, a number of Gram-negative bacterium have been discovered which secrete high molecular weight (100-110 kDa) calcium-dependent cytotoxic proteins which are immunologically and genetically related to the alpha-hemolysin (HlyA) of E. coli. These toxins have been designated the RTX (Repeat of Toxin) family on the basis of a series of glycine/aspartic acid-rich nonapeptide repeats found in the carboxylterminal third of the toxin protein (McWhinney et al., Separable Domains Define Target Cell Specificities of an RTX Hemolysin from Actinobacillus pleuropneumoniae, 1992, J. Bacteriol., vol. 174, pp. 291-297). The genetic determinants for the secreted RTX toxins consist of four genes: "A", the structural gene for toxin protein; "C", which is required for "activation" of the toxin prior to secretion; and "B" and "D", which are essential for the process of secretion. The four RTX genes are typically found in a single transcriptional unit, "CABD", and are expressed from a common promoter located upstream of the "C" gene (Felmleel et al., Nucleotide Sequence of an Escherichia coli Chromosomal Hemolysin, 1985, J. Bacteriol., vol. 163, pp. 94-105; Highlander et al., Secretion and Expression of Pasteurella haemolytica leukotoxin, 1990, J. Bacteriol., vol. 172, pp.2343-2350; and Mackman et al., Genetical and Functional Organization of the Escherichia coli Haemolysin Determinant 2001, 1985, Mol. Gen. Genet., vol. 201, pp.282-288). However, in Actinobacillus pleuropneumoniae determinants (AppII), the B and D gene pairs are lost during the evolutionary process (Chang et al., "The Actinobacillus pleuropneumoniae Hemolysin Determinant: Unlinked AppCA and AppBD Loci Flanked by Pseudogenes", 1991, J. Bacteriol.vol. 173, pp. 5151-5158).
A new species of organism has been isolated from pigs with enteritis, which is the basis of this application. The organism is a new member of the RTX family and has been characterized by Oberst et al. DNA-DNA hybridization studies showed that this organism could be distinguished from other organisms such as Pasteurella haemolytica (P. haemolytica), Pasteurella multocida (P. multocida), Escherichia coli, Pseudomonas aeruginosa, Actinobacillus pleuropneumoniae, and Salmonella choleraesuis . The proposed name for this new species of bacterium is "Pasteurella suis (P. suis)". P. suis secretes a leukotoxin with similar molecular weight to that of P. haemolytica leukotoxin (105 kDa). This leukotoxin is less species specific than that of P. haemolytica leukotoxin, because it kills both BL-3 cells and pig leukocytes. The gene or gene fragment encoding the novel leukotoxin secreted from P. suis can be used as immunogens in vaccine formulations to immunize pigs against diarrhea and as reagents in diagnostic assays.